Dew, Frost, and Fog
Humidity Humidity -Amount of water vapor in a given volume of air Absolute Humidity – The actual amount of water vapor in a given volume of air –in grams per cubic meter Relative Humidity – The percentage of water vapor in a given volume of air 100% Saturated (Full), it cannot hold any more water vapor - condenses
The warmer the air, the more water vapor it can hold The colder the air, the less water vapor it can hold So when air is cooled, it will be able to hold less water and can quickly become saturated
Relative Humidity
Dew and Frost Temperature quickly drops at night Cools air directly above surface Air molecule shrinks (Humidity increases) until air is saturated The dew point is the point at which the air becomes 100% saturated and vapour becomes liquid Water droplets form on grass = Dew If temperature is below freezing when this happens = Frost
Fog Air that is close to the Earth that reaches its dew point and begins to condense “Low lying cloud”
Radiation Fog Forms during clear and cool nights Ground cools air near surface When dew point is reached, condensation happens and fog will form. Does not form when there is wind, as the wind would cause the cool air to mix with warmer air
Radiation Fog
Radiation Fog
Advective Fog Occurs when moist air passes over a cool surface by advection (wind) and is cooled. Common when tropical air encounters cooler waters. Eg. California coast When warm air passes over a cold current, it can create advective fog
Advective Fog
Advective Fog
Advective Fog
What do you need to know about clouds? High Clouds – 7000-13000 meters Mostly ice crystals, not precipitation-forming Cirrus clouds are high, wispy clouds Cirrostratus and cirrocumulus take shapes like the types they are named after Stratus= wide, flat layers Cumulus= puffy
Low clouds –less than 3000 meters Stratus clouds – layers of grey cloud, usually cover most of the sky – can cause light precipitation Nimbostratus – dark, thick layers of cloud -produce lots of precipitation Stratocumulus –layers of puffy clouds, not much precipitation
Middle clouds -2000 to 7000 meters Altostratus – gray and layered, lots of moisture Altocumulus – white and grey, puffy
Vertical Clouds – generated by rising air currents (convection) -can reach great heights Cumulus- dark base, tall white clouds Cumulonimbus –air circulates quickly in this cloud, can produce lighting and hail -anvil shaped vertical cloud -associated with extreme weather
Orographic precipitation Warm, moist air is forced over a mountain As the air rises, it cools and condenses There is heavy rain on the windward side of the mountain On the leeward side, the air lacks moisture and often creates a rain shadow (area with little rain) Decreasing pressure creates adiabatic cooling Increasing pressure creates adiabatic heating
Frontal precipitation When 2 air masses meet, the warmer mass will be forced to rise As the air rises it cools and the moisture condenses, causing precipitation Warm fronts are when a warm mass moves over a colder mass and rises with a gradual slope Warm fronts bring precipitation over a large area
Cold Front A cold front is when a cold mass moves into a warm mass It forces the warm air up at a steep angle This creates vertical clouds (cumulonimbus) that can cause heavy, short periods of precipitation Cold fronts move faster than warm fronts and create more severe precipitation, such as hail
Convectional Precipitation Occurs in tropical areas or in the middle of large land masses The sun heats the ground, causing the warm air to rise As the air rises it cools and condenses, usually creating a cumulonimbus cloud These clouds are associated with short, heavy rains and sometimes hail and thunderstorms
Thunderstorms and Hailstorms Thunderstorms are formed when warm, moist air is forced upwards The rapid rising and cooling of air creates extreme precipitation and winds As water droplets are circulated in this rising and falling air, they freeze as they go up and are coated with more water as they go down They may continue to rise and fall, growing in size until they are so large that they fall to the ground
Intense storms can cause lightning and thunder Lightning is the discharge of electricity from the clouds to the surface of the earth or other clouds It is caused when the ice particles in the cloud rub against each other, forming an electric charge The lightning bolt is caused when the electricity moves from one polarity to the other
Lightning reaches immense heat, as high as 28000C This heat causes the surrounding air to expand rapidly, just like an explosive This explosion creates the sound waves that we hear as thunder
Assignment Explain the difference between relative humidity and aboslute humidity. Explain what dew point is and how it relates to the creation of clouds Explain how the two types of fog are formed. Explain how convection causes the creation of cumulonimbus clouds In your own words, explain the 4 types of precipitation we discussed. How does convection lead to the creation of hail and lightning?